Clean up Web UI multi import property editing

When importing multiple documents in Web UI and clicking add properties, you have the list of files.

Clicking "edit next" is supposed to carry all metadata over to the next file, except the title.

If you change the document type, the metadata from the previous document is still assigned to the current document even though they don't exist for the current document type. They are then submitted to the server, which could be error-prone.

Indeed, the changes done in NXP-25280 and NXP-27978 assumed that we never submit unrelated metadata when creating or editing a document and broke this Web UI feature. We fixed it with NXP-28349.

Note that we probably designed the multi import this way in order to cover use cases such as:
Given I have 3 files that I edit with the following order:

• doc1 -> Picture
• doc2 -> Note
• doc3 -> Picture
  When I edit doc3, Then I want to have the picture specific metadata I have filled in doc1

If this is really intended, we could probably do it in a cleaner way and do not submit inexisting metadata for a given doctype.

However, this can